The present invention relates to ion mobility spectrometers and more particularly to ion mobility spectrometers incorporating a body of vapour absorbent material such as a molecular sieve pack within the instrument housing.
It is well known that for correct operation at least the drift space of an ion mobility spectrometer cell needs to contain gas that is free of contaminants such as residual sample vapours. For correct operation there is also a need to maintain the level of moisture vapour, which may also be regarded as a contaminant, at well below that of ambient atmosphere. In prior art instruments such conditions are typically achieved by sweeping the drift space either with a flow of clean dry gas, which may come from an air-line, or by use of a complex re-circulating air system such as is described in U.S. Pat. No. 4,317,995.
However in a recent development, the subject of our co-pending application No WO93/06476, the necessary clean dry conditions are maintained within an ion mobility spectrometer by means of a diffusion process, in which a body of molecular sieve material is held in intimate contact with the internal volume of the IMS cell. As the barrier between cell and molecular sieve may only be a thin sheet of mesh, any contaminant vapours within the cell will diffuse towards the sieve pack where they are absorbed and the cell volume thus kept clean and dry.
The system described works well in many applications, particularly if the sample vapour to be analysed is drawn into the cell intermittently allowing time for residual sample vapour and any water vapour drawn in with it to diffuse to the molecular sieve pack and to be absorbed before the next sample is drawn into the cell. By obviating the need either for an external air supply or for a complex recirculatory system, use of a molecular sieve pack arrangement within the ion mobility spectrometer casing permits the design of small and simple ion mobility spectrometer instruments, ideally suited for use in miniature hand-held or body-worn IMS detectors.
However if such instruments are employed for continuous use, problems may arise as a result of the relatively long clear-down time, that is the time taken for the molecular sieve pack to cleanse the internal atmosphere of the cell to a level at which a further sample may be accurately measured.
Additionally, problems may be encountered with the absorbing capacity of the molecular sieve under conditions of continuous use. In the diffusion cleaning process, over a short timescale only those layers of the sieve nearest the IMS cell space are active, as the contaminant vapours do not reach the inner layers of the sieve. Consequently after a period of continuous use the absorbing capacity of the active layers becomes exhausted and the clear-down time of the instrument increases. If the instrument is then switched off, preventing the intake of any further sample, an equilibrium of the absorbed vapours within the molecular sieve pack occurs and the active layers become absorbent again.
Thus whilst a simple IMS instrument incorporating a molecular sieve pack and a vapour diffusion cleansing process is acceptable for intermittent or infrequent use, for example for periodic testing of an ambient atmosphere for trace of a possible contaminant or contaminants, it is less suitable for continuous use.
It is an object of the present invention to overcome the shortcomings of such an ion mobility instrument employing a vapour absorbent material, such as molecular sieve pack, in intimate contact with the drift space of the internal volume of the IMS cell, as the cleansing and/or dessicating means, whilst retaining the advantages offered by such an instrument.